<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>input-with-prefix</title>
    <style>
        .input-box {
            display: flex;
            align-items: center;
            max-width: 300px;
            background-color: #fff;
            border: 1px solid #a0a0a0;
            border-radius: 4px;
            padding-left: 0.5rem;
            overflow: hidden;
            font-family: sans-serif;
        }
        .input-box .prefix {
            font-weight: 300;
            font-size: 14px;
            color: #999;
        }
        .input-box .input {
            flex-grow: 1;
            font-size: 14px;
            border: none;
            outline: none;
            padding: 0.5rem;
            background-color: #fff;
        }
        .input-box:focus-visible {
            border-color: #777;
        }
    </style>
</head>
<body>
    <div class="input-box">
        <span class="prefix">+30</span>
        <input type="tel" class="input" placeholder="210 123 4567">
    </div>
</body>
</html>